Hey guys,
I’m using shadow catcher for my render, when I’m looking through the camera in render mode the shadow looks good, but when I render it it becomes much lighter, any ideas?
What version of Blender are you using?
Depending on your viewport and render settings, it’s possible the shadows may only appear lighter.
Do you have a background image and do you have it visible in both the viewport and the render? Are you rendering with transparency? If you’re rendering with transparency, you’re seeing dark shadows on a dark, checkered background. More info is needed. 
We need to see your blend-file. How did you rig-up the shadow catcher and then what are you doing with the shadow information. The shadow-catcher detects where the shadows are located. It’s easy to use a “curves” node to re-map the shadows, making them lighter or darker, or maybe giving them a bluish cast.
You can always do things like putting the alpha layer through curves in composting nodes to fix things like this. If that will effect the rest of your scene, you can use object index or material index so you can do this to just your shadow catcher. Material and object index are not in Eevee, though.
Thanks for your help guys, I’m going to try to remedy it with curves in composting and see how that works, then get back to you (with a half decent render hopefully).
On a related note, is it possible to have one object cast a shadow and another not? I have a grass shaped plane that will be off camera to the side, however I don’t want it to cast a shadow on the ground plane, but rather the car, I only want the car to cast the shadow on the ground plane (the shadow which I referred to at the beginning of this thread).
Hope that makes sense. 
It is. Figure out the Lightpath node, it has solutions to materials that are only visible under certian cercomstances.
Also, familiarize yourself with the various scene-diagnostic tools in Blender. For instance, the histogram tool, the various video analysis tools, and the tool which lets you point at a particular pixel and read its value. Your eyes will deceive you, but measurements will not.